What drives snowstorms and seasonal snowfall amounts in Colorado? Russ S. Schumacher Colorado State Climatologist Director, Colorado Climate Center Department of Atmospheric Science, Colorado State University Along with: Becky Bolinger, Peter Goble, Zach Schwalbe, Nolan Doesken Western Snow and Ice Conference 27 September 2018 Annual average precipitation Data: PRISM climate group prism.oregonstate.edu COLORADO CLIMATE CENTER Cheyenne Ridge Platte Valley Palmer Divide Arkansas Valley Raton Mesa http://www.joeandfrede.com/colorado/misc_trip_reports/colorado_topo_med_res.png COLORADO CLIMATE CENTER What are our data sources? COLORADO CLIMATE CENTER NWS Cooperative observer program – long-term measurements COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER National Weather Service automated stations Photo of an ASOS station Map showing NWS stations (from mesowest.utah.edu) COLORADO CLIMATE CENTER “Mesonets” Oklahoma Mesonet http://www.mesonet.org/ COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER CoAgMET ❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations ❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu Note: precipitation data is only good in the warm season! COLORADO CLIMATE CENTER CoAgMET ❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations ❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu COLORADO CLIMATE CENTER CoAgMET ❑ 75 stations ❑ 10 coming soon on west slope ❑ 44 5-minute stations ❑ interactive mapping through eRAMS ❑ includes ❑ time series charts ❑ site photos coagmet.colostate.edu National Weather Service automated stations Map showing NWS stations (from mesowest.utah.edu) COLORADO CLIMATE CENTER NWS stations + CoAgMET + other networks (from mesowest.utah.edu) COLORADO CLIMATE CENTER The problem with most of these networks: no reliable measurements of frozen precipitation! COLORADO CLIMATE CENTER SNOTEL (Snow telemetry) Tower SNOTEL site (10,500 ft, near Steamboat) https://www.wcc.nrcs.usda.gov/siteimages/825.jpg Note: not current data!! COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER Precipitation from the Tower SNOTEL site Nearly 60” of precipitation (liquid equivalent) each year! Almost all comes from snow at this elevation Plus the “old-fashioned way” A different kind of snowboard https://denver.cbslocal.com/2015/12/15/theres-more-to- measuring-snow-than-just-finding-out-how-many-inches- fell/ COLORADO CLIMATE CENTER Putting it all together… Data: PRISM climate group prism.oregonstate.edu COLORADO CLIMATE CENTER In every month, it’s the wettest month somewhere in Colorado! Data: PRISM climate group prism.oregonstate.edu COLORADO CLIMATE CENTER What do we want to know about the snow? • In Colorado, we’re usually most concerned about snow-water equivalent (SWE): this is what provides the water for rivers, reservoirs, agriculture, drinking water, etc., etc. • The general public mostly wants to know how deep it will be • But in your business, surely it’s not just the depth, but also other characteristics: – Wet or dry – How fast is it accumulating? – Drifting? • And these things are difficult to measure, and even harder to predict! COLORADO CLIMATE CENTER COLORADO CLIMATE The “forecast funnel” CENTER The “forecast funnel” • In weather forecasting, we are always thinking about the different “scales” (both time and space) that are important to whatever it is we’re trying to predict • We’ll usually start at the very large scales, and then “funnel” down to the smaller scales • Let’s use that approach today for understanding snowstorms… COLORADO CLIMATE CENTER Scales of motion in the atmosphere Planetary scale: broad, often slow-moving waves through the middle and high latitudes But fast-moving winds in the jet streams through those waves! earth.nullschool.net COLORADO CLIMATE CENTER Scales of motion in the atmosphere Synoptic scale: the jet stream, large storm systems (cyclones) earth.nullschool.net COLORADO CLIMATE CENTER Scales of motion in the atmosphere Mesoscale (middle scale): fronts; thunderstorms; interactions with mountain ranges and land/sea contrasts weather.cod.edu/satrad/exper COLORADO CLIMATE CENTER Scales of motion in the atmosphere Microscale: local effects owing to small variations in terrain; land cover; instabilities in the atmosphere; etc. Seasonal drivers: El Niño-Southern Oscillation (ENSO) • One of the biggest drivers of seasonal climate over North America is ENSO, the periodic variation of ocean temperatures in the Pacific Ocean • Its effect is to shift where the jet stream tends to be, how wavy it is, etc. • We are currently in neutral conditions, but most forecasts point to El Niño conditions being in place by late fall/early winter COLORADO CLIMATE CENTER COLORADO Example of a strong El Niño (1997-98) CLIMATE CENTER Seasonal drivers: El Niño-Southern Oscillation (ENSO) El Niño refers to the waters in the central and eastern Pacific being warmer than normal (La Niña is when waters in this region are cooler than normal). In turn, this affects where large thunderstorm complexes develop in the tropics, which then affects the jet stream pattern over North America COLORADO CLIMATE CENTER Seasonal drivers: El Niño-Southern Oscillation (ENSO) El Niño refers to the waters in the central and eastern Pacific being warmer than normal (La Niña is when waters in this region are cooler than normal). In turn, this affects where large thunderstorm complexes develop in the tropics, which then affects the jet stream pattern over North America COLORADO CLIMATE CENTER Seasonal drivers: El Niño-Southern Oscillation (ENSO) El Niño refers to the waters in the central and eastern Pacific being warmer than normal (La Niña is when waters in this region are cooler than normal). In turn, this affects where large thunderstorm complexes develop in the tropics, which then affects the jet stream pattern over North America What about for Colorado? • We sit in between the more robust signals of warm/dry to the north, and cool/wet to the south • Therefore, the ENSO phase just isn’t that reliable of a predictor for our seasonal snowfall totals – If the jet stream position (on average) pushes slightly one direction or the other, it can have a big influence on our winter! • Last winter was La Niña conditions, and it turned out to follow the usual pattern: relatively cool and snowy to the north, and warm and dry to the south • But El Niño winters seem to have even more variability... COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER Correlation of ENSO phase to seasonal snowfall Fort Collins: a slight tendency for snowier winters during El Niño, but lots of variability COLORADO CLIMATE CENTER Correlation of ENSO phase to seasonal snowfall Similar at Denver: slight tendency for snowier winters during El Niño, but lots of variability COLORADO CLIMATE CENTER Correlation of ENSO phase to seasonal snowfall In most mountain areas, the correlation is even weaker At Copper Mountain, a very slight tendency toward less-snowy winters during El Niño COLORADO CLIMATE CENTER Correlation of ENSO phase to seasonal snowfall The one mountain area with a reasonably strong signal is in the Sangre de Cristos, which usually see more snow in El Niño winters COLORADO Averaged over the Upper Colorado River Basin CLIMATE CENTER Wet years tend to come during El Niño, but it’s no guarantee... ENSO phase “A case study of a wet and a dry year (with similar ENSO/PDO conditions) shows that the occurrence of a few large accumulating events is what drives the seasonal variability” ENSO phase -Bolinger et al. (2014, Journal of Climate) COLORADO CLIMATE CENTER What about big snowstorms along the Front Range? At Fort Collins (and similar for other Front Range locations), big snowstorms occur more frequently during El Niño winters, but the relationship isn’t particularly strong El Niño summary • The phase of ENSO does slightly tilt the odds in terms of snow seasons in Colorado: – Southern Colorado tends to be cooler and snowier than average – The Front Range and Plains are somewhat more likely to see a big snowstorm – The relationship in the central mountains is not especially strong • But there are many more factors than ENSO that affect the frequency of snowstorms, and the seasonal snow accumulations in Colorado (as opposed to north and south of us, where connections are stronger) COLORADO CLIMATE CENTER COLORADO CLIMATE CENTER Winter outlook from Climate Prediction Center December-January-February outlook shows slightly increased chances of warmer and wetter conditions compared to normal COLORADO CLIMATE CENTER Winter outlook from Climate Prediction Center December-January-February outlook shows slightly increased chances of warmer and wetter conditions compared to normal COLORADO CLIMATE CENTER Synoptic-scale drivers: how does a big Front Range snowstorm come together? Let’s look at the March 2003 snowstorm: the biggest one in recent memory Top: upper- level winds and vorticity (“spin”) March 14 (3 days before): Warm on the plains, no sign yet of the storm Bottom: lower- at upper levels level winds and temperature Top: upper- level winds and vorticity March 15 (2 (“spin”) days before): Still warm on the plains, the first signs of the wave that will develop Bottom: lower- into the big level winds storm and temperature Top: upper- level winds and vorticity March 16 (1 (“spin”) day before): Still warm on the plains, wave just starting to develop Bottom: lower- level winds and temperature Top: upper- level winds March 17 (day and vorticity